Polyolefins can be prepared with variety of steric configurations that correspond to the manner in which each new monomer unit is added to a growing polyolefin chain. Four basic configurations are commonly recognized for polyolefins, atactic, hemi-isotactic, isotactic and syndiotactic. Of course, a given polymer may incorporate regions of each steric configuration, yet not exhibit the pure or nearly pure configuration.
Atactic polymers exhibit no regular order of repeat units in the polymer chain, i.e., the substituents neither alternate nor maintain the same orientation relative to a hypothetical plane containing the polymer backbone (the plane is oriented such that the substituents on the asymmetric carbon atoms are either above or below the plane), but assume a random distribution of orientations. On the other hand, isotactic, syndiotactic, and hemi-isotactic polymers have unique and regularly repeating stereochemistries.
The isotactic structure is defined as having the substituents attached to the pseudo-asymmetric carbon atoms with the same relative steric configuration. In a Fischer projection with the polymer backbone portrayed in the plane of the paper (the horizontal line), the vertical line segments representing the substituents all appear on the same side of the polymer as represented by the horizontal line: ##STR1##
Another way of describing the isotactic structure is through the use of NMR. In Bovey's NMR nomenclature, an isotactic pentad is represented by . . . mmmm . . . with each "m" representing a "meso" dyad or successive substituents on the same side in the plane. As is well known in the art, any deviation, disruption, or inversion about a pseudo-asymmetric carbon in the chain will lower the degree of isotacticity and crystallinity of the polymer.
In contrast, the syndiotactic structure is typically described as having the substituents, that are attached to the pseudo-asymmetric carbon atoms, pseudo-enantiomorphically disposed, i.e., the substituents are oriented alternately and regularly above and below the main polymer chain. In a Fischer projection, the substituents alternate above and below the polymer backbone: ##STR2##
In NMR nomenclature, a syndiotactic pentad is represented by . . . rrrr . . . in which each "r" represents a "racemic" dyad, i.e., successive substituents on alternate sides of the plane. The percentage of r dyads in the chain determines the degree of syndiotacticity of the polymer.
There are other variations in polymer structures as well. One such variant is the so-called hemi-isotactic polymers. Hemi-isotactic polymers are ones in which every other pseudo-asymmetric carbon atom has its substituent oriented on the same side relative to the plane containing the polymer backbone in a Fischer projection. While, the other pseudo-asymmetric carbon atoms can have their substituents oriented randomly either above or below the plane. Since only every other pseudo-asymmetric carbon is in an isotactic configuration, the term hemi is applied. In a Fischer projection, the substituent configurational sequence for a hemi-isotactic polymer is shown below: ##STR3##
Isotactic and syndiotactic polymers are crystalline polymers and are insoluble in cold xylene. Crystallinity distinguishes both syndiotactic and isotactic polymers from hemi-isotactic or atactic polymers that are soluble in cold xylene and non-crystalline. Atactic polymers are typically a waxy product and not crystalline. While it is possible for a catalyst to produce all four types of polymers (atactic, hemi-isotactic, isotactic and syndiotactic), it is desirable for a catalyst to produce predominantly or essentially exclusively isotactic or syndiotactic polymer with very few to essentially no defects.
Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. Nos. 4,794,096 and 4,975,403, as well as European Pat. Appln. 0,537,130. Catalysts that produce syndiotactic polyolefins are disclosed in U.S. Pat. Nos. 3,258,455, 3,305,538, 3,364,190, 4,892,851, 5,155,080, and 5,225,500. Cationic metallocene catalysts are disclosed in European Patent Applications 277,003 and 277,004. Catalysts that produce hemi-isotactic polyolefins are disclosed in U.S. Pat. Nos. 5,036,034.
Although the catalysts described in the patents listed above are capable of generating polymers that have a relatively high stereoselectivity to a given tacticity, all are subject to various phenomena that introduce defects in the stereoregularity of the polymers. One common defect in the preparation of isotactic polymers is the occasional introduction of the wrong orientation disrupting the isotactic placement of monomer units, as shown below: ##STR4##
On the other hand, an analogous defect in syndiotactic polymers (i.e., the occasional introduction of the wrong orientation) is the so-called meso triad defect, shown below: ##STR5##
Thus, it would be a significant advancement in the art to be able to design and prepare stereospecific catalysts capable of generating stereoregular polymers essentially free of defects or where the amount and type of defects can be statistically controllable through catalyst design. Polypropylene with higher tactiospecificity will have a higher melting point and, therefore, will better suited for higher temperature applications such as the fabrication of ironable zippers for the clothing industry. Of course, other uses will be apparent as well.